1. Technical Field
The present invention relates to a compact and economical solid oxide electrolyte fuel cell which comprises a simple structure to provide a large power generation area, and the thermal stress generated therein is low to ensure reliable fuel cell characteristics.
2. Background Technology
Conventionally, as a planar type solid oxide electrolyte fuel cell (hereinafter SOFC) in which power generation films comprise a dimpled structure (hereinafter "dimples"), the configuration shown in FIGS. 15 and 16 is known.
FIG. 15 is an exploded perspective view of the SOFC and FIG. 16 is a sectional view taken along line X--X in FIG. 15. In these figures, reference numeral 1 denotes an interconnector (also called a gas separator); 2, a power generation film, which together with the interconnector 1 constitute a unified fuel cell structure 10 (hereinafter "stack") in such a manner that from top to bottom an interconnector 1, a power generation film 2, an interconnector 1, and so on, are alternately superposed.
The power generation film 2 is the smallest unit cell (also called a cell) that constitutes a SDFC, and is mainly composed of a solid electrolyte film 20 comprising convex dimples 21 and concave dimples 22 substantially all over its surfaces, an oxygen electrode 23 on one side of the solid electrolyte film 20, and a fuel electrode 24 on the other side thereof.
In the SOFC shown in FIGS. 15 and 16, the side having convex dimples 21 is the oxygen electrode 23 and the side having concave dimples serves as the fuel electrode 24. The above power generation film 2 is surrounded by a seal material 3 in the circumference thereof, except for gas inlet and outlet apertures through which oxidant gases (e.g. air, hereinafter air) and fuel gases pass, and each film is sandwiched between two interconnectors 1 to form air flow passages 41 and fuel gas flow passages 42.
The above interconnectors 1, on the other hand, are connected with the seal material 3 to provide space between themselves and the adjacent power generation film 2 and constitute gas flow passages, while providing electrical functions for series connection by contacting or connecting with the dimple protrusions of the adjacent power generation films.
The SOFC stack 10 thus constituted is then kept in a high temperature range of 800.degree. C..about.1000.degree. C., and power is generated when air flows through the air passages 41 and a fuel gas through the gas passages 42, respectively, as illustrated in FIGS. 15 and 16.
In the example in FIG. 15, the air flow and the fuel gas flow are perpendicular to each other on the top and the bottom side of each power generation film 2. For this reason, a SOFC having this kind of gas flow layout is generally called a cross flow type.
In the cross flow type shown in FIG. 15, an air inlet 43 is provided on one of the four sides of each planar type cell and an air outlet 44 is constituted on the opposite side thereof, whereas a fuel gas inlet 45 is given on one of the remaining sides and a fuel outlet (not shown) is prepared on the facing side thereof.
On the other hand, there is a planar cross flow type SOFC, in which each power generation film 2 is a flat plate without any dimple and grooves are provided in the interconectors 1 to constitute gas flow passages. A typical example of this configuration is shown in FIGS. 17 and 18.
FIG. 17 is an exploded perspective view of the said SOFC; FIGS. 18(a) and 18(b) are sectional views taken along line X--X and line Y--Y in FIG. 17, respectively
Each power generation film 2 is a flat plate without any dimple, and is composed of a solid electrolyte film 20, an oxygen electrode 23 on one side of the power generation film 20, and a fuel electrode 24 on the other side thereof in FIG. 18.
In FIG. 17, reference numeral 1 denotes an intermediate interconnector on both sides of which multiple rows of grooves 33 for gas flow are provided along the direction of gas flow. Reference numerals 1c and 1d in the same figure indicate an upper interconnector and a lower interconnector of the stack 11, respectively. On the surface facing a power generation film 2 of each of these interconnectors, multiple grooves 33 are provided along the direction of gas flow, and the opposite surface thereof is usually flat to fit power collecting parts for taking out electric current. The interconnectors 1, the upper interconnector 1c, and the lower interconnector 1d alternately isolate the power generation films 2, thereby forming air flow passages 41 and fuel gas flow passages 42 between themselves and the adjacent power generation films 2, and having at the same time functions for electrical series connection by contacting or connecting with the protrusions 32 of the interconnectors and the oxygen electrodes 23 as well as the fuel electrodes 24 of the adjacent power generation films.
In FIG. 17, the SOFC stack 11 is composed of an upper interconnctor 1c, a power generation film 2, an intermediate interconnctor 1, a power generation film 2, . . . , a lower interconnector 1d, which are superposed alternately from top to bottom, thus constituting a unified structure. The stack 11 is then kept in a temperature range of 800.degree. C..about.1000.degree. C., and power is generated, as shown in FIG. 18, by letting air flow through the air flow passages 41 and a fuel gas through the fuel gas flow passages 42.
In a SOFC, the operation temperature is as high as 800.degree. C..about.1000.degree. C., and the reaction in the fuel cell generates heat. As a result, the temperature distribution in the fuel cell is such that the area near the gas inlet is in a low temperature range and the area near the gas outlet is in a high temperature range. In the SOFC's as shown in FIGS. 15.about.18 in which the gas flow occurs according to the cross flow method, a temperature distribution as shown in FIG. 20(a) is observed. The % values in FIG. 20(a) indicate approximate ratios when the temperature difference between the gas inlet and the gas outlet is regarded as 100%. Once such a temperature distribution occurs, thermal stress is generated in each part of the fuel cell. If the thermal stress becomes too high, the heat build-up associated with the cell reaction increases by taking out much output, for example, and an excessive temperature difference between the gas inlet and the gas outlet results in a higher thermal stress, thus causing the electrical connection between the stacked cells to deteriorate partially, or damaging the surrounding gas seal parts to cause a decrease in power generation capability, which in some cases could lead to fractures of the interconnectors or the power generation films. In such cases, the expected power output cannot be obtained and the function as a fuel cell itself may be lost.
One of the means to avoid such trouble is to decrease the temperature difference between the gas inlet and the gas outlet of the fuel cell by providing much air to remove the reaction heat of the cell, thus maintaining reliable characteristics of the fuel cell.
However, such a method requires high ventilating power to send a large volume of air, as well as a large-sized heat exchanger or heater in order to preheat the large volume of air up to a temperature close to the operating temperature of the SOFC. As a result, the fuel cell becomes uneconomical as a power generation unit.
On the other hand, as a structural means to solve the above problem, there is the so-called co-flow method by which the air and the fuel gas flow parallel to each other in the same direction.
A typical example which employs this method is shown in FIGS. 21 and 22. In these figures, reference numeral 5 denotes a header; 6, a gas pre-flow rectifying section. Both 5 and 6 are provided as rectifying sections for the air or the fuel gas to flow uniformly in one direction. The other reference numerals are the same as those explained in FIGS. 15, 16 and 17, 18.
In the SOFC illustrated in FIGS. 21 and 22, in which the co-flow method is employed, a temperature distribution as shown in FIG. 20(b) is observed. In such a fuel cell, the temperature increases gradually from -he gas inlet toward the gas outlet of the cell, enabling relatively free thermal expansion with small self-constraint. As a result, the thermal stress caused by the heat build-up in the cell becomes also small. In summary, under the same condition, the heat distribution that occurs in the cell of the co-flow type shown in FIG. 20(b) results in lower thermal stress than in the cell of the cross flow type in FIG. 20(a), thus improving the fuel cell characteristics and providing full performance of the fuel cell.
By the co-flow method, however, it is necessary to prepare two kinds of gas inlet apertures 43, 45 for air and fuel gases, respectively, or outlet apertures 44, 64 on one of the four sides of the planar fuel cell, which requires a more complicated manifold structure for gas inlet and outlet. As a result, the co-flow method becomes inferior to the cross flow method in terms of reliability and economy.
In order to obtain the temperature distribution as shown in FIG. 20(b), it is also necessary to let the gases flow uniformly in one direction. To realize this goal, the header 5 as shown in FIG. 21 or the gas pre-flow rectifying section 6 as shown in FIG. 22 has been invented. For a uniform gas flow, the header (reference numeral 5 in FIG. 21) and the gas pre-flow rectifying section (reference numeral 6 in FIG. 22) need to be wide enough, which does not actually contribute to the power generation itself, resulting in a relative decrease in the effective power generation area of the fuel cell (i.e. in the power generation film in FIGS. 21 or 22, the area contributing to the power generation is only the hatched part that comprises the oxygen electrode 23 on one side and the fuel electrode 24 on the other side). As a result, the fuel cell becomes larger in order to obtain a desired power output, and less economical as compared with the cross flow method.
As explained above, by -he cross flow method, each of the air flow and the fuel gas flow are almost uniform in one direction, and the cell reaction is efficient by using nearly the entire surface of the power generation film, whereas high thermal stress occurs, which may lead to unreliable cell characteristics. By the co-flow method, on the other hand, less thermal stress occurs, and therefore better cell characteristics can be obtained as compared with the cross flow method. However, a header or a gas pre-flow rectifying section which does not contribute to power generation becomes necessary in order to realize an ideally uniform parallel gas flow in one direction for a sufficient cell performance, thus inevitably requiring a larger fuel cell area to obtain a desired power output, which makes the co-flow method less economical. For this reason, required is a fuel cell structure which compensates the disadvantages of both of the above-mentioned gas flow methods.
In view of the above problems, an object of the present invention is to provide a compact and economical SOFC in which the thermal stress is low to realize reliable cell characteristics and the structure is simple to ensure a wide power generation area as is the case with a SOFC by the cross flow method.